The present invention relates to an image reading apparatus, and in particular to an image reading apparatus capable of correctly extracting an image which has any tone without using a special back pad for inclination correction or extraction of an image.
A related image reading apparatus must read a white reference in order to perform white correction or shading correction. For this purpose, the white reference is set by using a back pad having a white color for an original and reading the white color of the back pad.
Actually, the white color of the back pad does not have a constant gradation but has a gradation characteristic varying in vertical direction as shown in FIG. 1.
In case an edge of an original is extracted based on a variation amount of contrast difference between the original and the back pad per pixel in a line in the main scan direction as in a related edge detection method, setting a too small value as the threshold of the variation amount for determining the edge, a part that is not the edge is determined as the edge. Setting a large value as the threshold, the edge cannot be detected in case the background in the original is white and the contrast difference is a small.
For example, referring to FIG. 1, assuming that the variation amount of the contrast difference to be determined as an edge is 5-step gradation, the area corresponding to 296 and 297 pixels is erroneously determined as the edge although it is actually the back pad. Assuming that that the variation amount of the contrast difference to be determined as an edge is 10-step gradation, in case the contrast difference between the white background in the original and the white color of the back pad is small, for example, difference of 9-step gradation, the edge detection for the original is not performed.
In order to solve such a problem, a device has been conceived that includes a mechanism to switch between a white back pad and a black back pad and that uses the white back pad in reading a white reference and using the black back pad in reading an original thus increasing the contrast difference between the background color in the original and the color of the back pad, thereby correctly extracting the image of the original (for example, JP-A-2002-300367).
Such a device must be equipped with a mechanism to switch over the color of the back pad. This adds to the size of the device or complicates the mechanism or control.